Recently, there is an increasing interest in energy storage technology. Batteries have been widely used as energy sources in portable phones, camcorders, notebook computers, PCs and electric cars, resulting in intensive research and development into them. In this regard, electrochemical devices are subjects of great interest. Particularly, development of rechargeable secondary batteries is the focus of attention. Recently, research and development into novel electrode and battery that can improve capacity density and specific energy have been made intensively in the field of secondary batteries.
Among currently used secondary batteries, lithium secondary batteries appearing in early 1990's have drive voltage and energy density higher than those of conventional batteries using aqueous electrolytes (such as Ni—MH batteries, NI—Cd batteries, H2SO4—Pb batteries, etc). For these reasons, lithium secondary batteries are advantageously used. However, such lithium secondary batteries have disadvantages in that organic electrolytes used therein may cause safety-related problems resulting in ignition and explosion of the batteries and that processes for manufacturing such batteries are complicated. More recently, lithium ion polymer batteries developed for the purpose of overcoming the shortcomings of lithium ion batteries have been thought of as a candidate leading the next generation batteries. However, such lithium polymer batteries developed up to date have a relatively low capacity compared to lithium ion batteries and provide insufficient discharge capacity at low temperature. Therefore, there is an imminent need for batteries capable of solving the above-mentioned problems.
Lithium ion batteries have an operation mechanism different from that of nickel-metal hydride batteries or nickel-cadmium batteries. Each of LiCoO2 and graphite used in a lithium ion battery as cathode active material and anode active material, respectively, has a crystal structure in which an empty space is present. During charge/discharge cycles, Li ions repeatedly intercalate into and deintercalate out of the empty space and thus move inside of a battery.
A battery is manufactured in its discharged state. During a charge cycle, lithium contained in the LiCoO2 crystals deintercalates out of the crystals, moves to an anode and thus intercalates into the crystal structure of graphite. On the contrary, during a discharge cycle, lithium contained in graphite deintercalates out of the crystal structure of graphite and then intercalates into crystals present in a cathode. Such repeated comings and goings of Li ions between a cathode and anode are referred to as the so-called rocking chair concept, which forms the operation mechanism of a lithium ion battery.
Evaluation of and security in safety of batteries are very important. It should be considered in the first place that users have to be protected from being damaged due to malfunctioning of batteries. To satisfy this, safety of batteries is strictly restricted in terms of ignition and combustion in batteries by safety standards. Overcharge of a battery is the most imminent problem to be solved.
All batteries are dangerous when overcharged and lithium ion batteries cannot be an exception. When a battery is overcharged, lithium ions move continuously from a cathode to an anode present in a state wherein lithium completely occupies the empty space in the crystal structure of graphite, as viewed from the geometrical point, so that lithium ions grow on the surface of anode, resulting in formation of dendrite having a resinous structure. Such dendrite may result in explosion and firing of a battery when the battery is abused. Morphology of the dendrite depends on the kind of lithium salt contained in an electrolyte.
The most dangerous phenomenon resulting from overcharge of a battery is “high-temperature overcharge”, which is the worst case occurring in lithium ion batteries. When a lithium ion battery is overcharged to a voltage of 4.2V or more, electrolyte starts to be decomposed and tends to have a high possibility for ignition as the battery temperature increases to reach the flash point. However, there is no occurrence of ignition in the closed spaced of a battery because oxygen is not supplied thereto. LiCoO2 used as cathode active material forms a layered structure of “O—Co—O” in which a Co layer locates between oxygen atom layers, such structure forming a sandwich-like shape. Additionally, LiCoO2 may form a crystal structure of “O—Co—O—Li—O—Co—O” in which a Li layer locates between two sandwich-like structures. The latter structure is not stable.
At high temperature, LiCoO2 has a great tendency to be converted into a stable spinel structure (die-like structure). The spinel has a molecular formula of LiCO2O4 and thus has a small amount of oxygen per unit cell compared to a layered structure. Therefore, in this case, remaining oxygen moves to an electrolyte so that oxygen may be supplied to the electrolyte reaching its flash point, thereby causing explosion of a battery. However, because a battery itself cannot prevent the heat emission as mentioned above, many attempts have been made, for example, to mount a protection circuit on a battery or to apply heat obstruction by using a separator.
Particularly, it is known that protection devices such as a PTC (positive temperature coefficient) device or thermal fuse are efficient when they are disposed in the vicinity of an electrode as heat emitting source (for example, at the central portion or lateral surface of a battery) by means of resistance welding, in order to promptly detect an increase in battery temperature followed by abnormal operation of the battery. Additionally, such protection devices are frequently disposed at the lateral side portion of a battery so as to increase energy efficiency per volume.
As the most recent approach, Japanese Laid-Open Patent No. 2003-45492 discloses a battery comprising a heat-sensitive protection device (PTC) mounted on an electrode lead having relatively high heat conductivity, wherein the corresponding protection device is disposed at the sealing region. However, according to the battery, because the PTC device is mounted on the exterior of a battery and the battery casing has low heat conductivity, it is not possible to respond sensitively to variations in temperature inside of the battery in practice. Further, because the battery is manufactured through a complicated process, it shows poor industrial applicability in practice.